A Sikh boy has a turban tied over his head during a gathering for World Turban Day. NARINDER NANU/AFP/Getty Images

Law enforcement authorities don't know why suspected gunman Wade Michael Page burst into a Sikh temple in Oak Creek, Wis., on Sunday and opened fire, killing six before he was shot dead by the police.

But many American Sikhs say they do know this: Their community has been targeted by a growing number of hate crimes since the 9/11 terrorist attacks. The New York-based Sikh Coalition reports more than 700 such incidents since 2001. The question is: Why?

"The turban is the main issue here," said Pashaura Singh, a professor of Sikh and Punjabi studies at the University of California-Riverside. "People confuse Sikhs with Osama bin Laden."

Following 9/11, bin Laden and his al-Qaida associates were often shown in media reports wearing white turbans. Combine that with a lack of basic knowledge about the estimated 500,000 Sikhs living in the U.S. and you get tragic — and sometimes violent — cases of mistaken identity.